1. Field of the Invention
This invention relates in part to improvements in methods and apparatuses for dynamic information storage or retrieval, and more particularly to improvements in methods and circuitry for positioning a transducer for writing or detecting data written onto a spinning data disk, and still more particularly to improvements in circuits for driving piezo-based milli-actuator structures and methods for making same.
2. Relevant Background
Mass data storage devices include well known hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected further to increase in the future. In the construction of mass data storage devices, a data transducer, or head, is generally carried by an arm that is selectively radially positionable by a servo motor. Recently, so-called milli-motors, or milli-actuators, have been considered to provide better, or more accurate, position control of the head. A milli-actuator is generally constructed with a piezo element carried by the positionable arm and to which the head is mounted. A current is selectively applied to the piezo element, which causes the piezo element to deform, thereby moving the head a small, controllable amount. This provides a fine adjustment to the position of the head. As track densities become denser, control of the position of the head becomes more critical. Thus, piezo-based milli-actuators are becoming of increasing importance in the head positioning mechanisms.
At least discrete circuits are available for providing drive signals to milliactuators to control the position of the head of the device or drive with which the milli-actuator is associated. Typically, the milli-actuator drive circuits operate by supplying a control voltage to the piezo element of the milli-actuator.
Although such voltage mode circuits are relatively easy to build, they have several problems. First, the deformation response of piezo elements generally is highly temperature dependent. Thus, significant temperature compensation or calibration circuitry must be provided to assure accurate head positioning over the range of expected operating temperatures of the drive. Secondly, relatively high voltages are required to operate the piezo elements, which may require relatively large circuit components, and may complicate the overall circuit design. Thirdly, piezo elements generally have strong hysteresis effects.
As a result, charge mode milli-actuator circuits have been proposed. Charge mode milli-actuator circuits typically have a capacitor in series with the piezo element, such that a charge builds up on the capacitor that is proportional to the charge on the piezo element. The change in voltage across the capacitor is measured in a given time, and the product of the measured voltage change times the capacitance of the capacitor equals the charge on the capacitor. This value can then be used to adjust the charge supplied to the piezo element. The charge mode technique is still subject to temperature variations and hysteresis effects, but these effects are substantially reduced, and, as a result, the charge mode of operation is more accurate than the voltage mode of operation. On the other hand, the piezo elements are typically large, having capacitances in the range in the thousands of picofarads. Thus, the capacitor that must be used must be proportionately large. Also, the charge used to charge the capacitor is unavailable for charging the piezo element.
One way in which at least some of these problems in the charge mode of operation have been addressed uses a mirror circuit technique in which the milli-actuator circuit provides a 1xc3x97 mirror circuit connected to a sense capacitor. An nxc3x97 mirror circuit mirrors the current in the 1xc3x97 mirror circuit to drive the piezo element. Thus, as the charge on the capacitor changes, the nxc3x97 output proportionally changes. Thus, at least the size of the sense capacitor can be reduced.
Thus, piezo elements may be driven in voltage or charge mode with advantages for both drive modes. When integrating the two modes into a single amplifier, the compensation of the amplifier must adapt to the selected mode. In addition, when in charge mode, the gain of the 1xc3x97 closed loop amplifier must be modified depending on the number of piezo elements being driven when a fixed sense capacitor is used. Since the closed loop gain of the 1xc3x97 amplifier may change from gain of one to gain of ten or more, the amplifier requires adjustable compensation so that a uniform bandwidth can be achieved and the amplifier remains stable.
What is needed, therefore, is a milli-actuator driver circuit that can operate at preestablished voltages other than the full rail voltages of the servo.
The solution to the problem is two-fold. To handle both a voltage mode solution and a charge mode solution, the compensation for each mode is different. In voltage mode, a feedback resistor in series with a capacitor is switched from the output of a second stage class AB amplifier to the output of a first stage class A amplifier. However, in the charge mode of operation, this feedback is not used and is switched to ground, and the amplifier is completely load compensated by the capacitor on the 1xc3x97 output. But in charge mode, the gain of the 1xc3x97 loop changes depending on the number of piezo elements driven which in a typical hard disk drive may be ten or more. Therefore, the output impedance of the first stage class A amplifier in conjunction with the capacitance seen on that node sets a pole that may inhibit stability at lower closed loop gains. To adapt to the lowering of the closed loop gain, the output resistance of the first stage amplifier is lowered to move the problematic pole out in frequency and keep the amplifier stable. It also keeps the bandwidth versus the closed loop gain mode relatively constant and thus the noise BW of the driver more constant. The output resistance is modified in the current implementation with MOSFETS with their gates tied to drains, however, this could also be accomplished with resistors as well.
What is different about the solution of the present invention, as compared with other solutions to the same problem, is that other solutions employ adjustable compensation, but it is not believed that any previous solutions integrate the two modes of operation and allow for a varying gain change.
Thus, one of the advantages of the present invention is that a single amplifier serves both voltage mode and charge mode drive while also allowing for a varying number of elements to be driven in charge mode where the closed loop gain must change to adjust for the load changes. In all cases, a single amplifier can be allowed to serve these multiple purposes and remain stable.
According to a broad aspect of the invention, an integrated circuit for providing drive signals to a piezo element of a milli-actuator device in a mass data storage device is provided. The device includes a driving circuit for selectively driving the piezo element in either a voltage mode or a charge mode and a circuit for compensating the driving circuit for a variable number of piezo elements in a charge mode of operation and providing a compensating feedback signal in a voltage mode of operation. Preferably, the circuit for compensating the driving circuit for a variable number of piezo elements in a charge mode of operation comprises a circuit for adjusting an output impedance of at least a portion of the driving circuit.
According to another broad aspect of the invention, an integrated circuit for providing drive signals to a piezo element of a milli-actuator device in a mass data storage device is provided. The piezo element includes a variable number of piezo element devices. A Class A amplifier is connected to receive input signals for controlling the piezo element and a Class AB amplifier is connected to receive an output from the Class A amplifier to selectively provide either current mode driving signals or voltage mode driving mode signals to the piezo element. A circuit for compensating the integrated circuit is provided, wherein in a charge mode of operation, the circuit selectively compensates the Class A amplifier for a variable number of piezo element devices, and in a voltage mode of operation, the circuit provides a compensating feedback signal. Preferably, the circuit for compensating the driving circuit for a variable number of piezo elements comprises a circuit for adjusting an output impedance of the Class A amplifier.
According to yet another broad aspect of the invention, a method for providing drive signals to a piezo element of a milli-actuator device in a mass data storage device is provided. The piezo element is of the type that includes a variable number of piezo element devices. The method includes selectively driving the piezo element in either a voltage mode or a charge mode, and compensating the driving circuit for the variable number of piezo element devices in a charge mode of operation and providing a compensating feedback signal in a voltage mode of operation. Preferably, compensating the driving circuit for a variable number of piezo element devices in a charge mode of operation comprises adjusting an output impedance of at least a portion of the driving circuit.
According to still yet another broad aspect of the invention, an integrated circuit for providing drive signals to a piezo element of a milli-actuator device in a mass data storage device is provided. The piezo element includes a variable number of piezo element devices. The integrated circuit includes means for selectively driving the piezo element in either a voltage mode or a charge mode, and means for compensating the driving circuit for the variable number of piezo element devices in a charge mode of operation and providing a compensating feedback signal in a voltage mode of operation. Preferably, the means for compensating the driving circuit for a variable number of piezo element devices in a charge mode of operation comprises means for adjusting an output impedance of at least a portion of the driving circuit.
According to still another broad aspect of the invention, a mass data storage device is provided. The mass data storage device includes an integrated circuit for providing drive signals to a piezo element of a milli-actuator device in a mass data storage device, and a driving circuit for selectively driving the piezo element in either a voltage mode or a charge mode. A circuit is provided for compensating the driving circuit for a variable number of piezo elements in a charge mode of operation and providing a compensating feedback signal in a voltage mode of operation. Preferably, the circuit for compensating the driving circuit for a variable number of piezo elements in a charge mode of operation comprises a circuit for adjusting an output impedance of at least a portion of the driving circuit.